Finding the origin of an arrythmia

ABSTRACT

A probe generates location signals, and has an electrode at a distal end which acquires from heart chamber surface positions electrical signals due to a conduction wave traversing the surface. A processor derives LATs from the electrical signals, calculates a first time difference between LATs at a first pair of positions and a second time difference between LATs at a second pair of positions. The processor calculates first and second LAT-derived distances as products of the first and second time differences with a conduction wave velocity, identifies an arrhythmia origin at a surface location where a first difference in distances from the location to the first pair of the positions is equal to the first LAT-derived distance, and a second difference in distances from the location to the second pair of the positions is equal to the second LAT-derived distance, and marks the origin on a surface representation.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims the benefit of U.S. Provisional PatentApplication 62/771,360 filed Nov. 26, 2018, which is incorporated hereinby reference.

FIELD OF THE INVENTION

The present invention relates generally to invasive medical procedures,and specifically to analyzing results of a cardiac invasive procedure.

BACKGROUND OF THE INVENTION

One type of cardiac arrythmia is a single-source arrythmia where thearrythmia has a localized origin. In order to cure the arrythmia, it isimportant to find the localized origin. Typically, once the origin hasbeen located, ablation may be applied to the origin, or to a region inproximity to the origin, to cure the arrythmia.

SUMMARY OF THE INVENTION

An embodiment of the present invention provides medical apparatus,consisting of:

a probe, configured to be inserted into a chamber of a heart of a humansubject, and to generate location signals indicative of locationcoordinates of a distal end of the probe, and including an electrode atthe distal end, which is configured to acquire from positions on asurface of the chamber respective electrical signals in response to aconduction wave traversing the surface;

a display; and

a processor, configured to:

process the location signals to generate and present on the display athree-dimensional (3D) representation of the surface,

derive respective local activation times (LATs) from the respectiveelectrical signals at the positions on the surface,

calculate a first time difference between the respective LATs at a firstpair of the positions on the surface and a second time differencebetween the respective LATs at a second pair of the positions on thesurface,

calculate first and second LAT-derived distances as respective productsof the first time difference and the second time difference with aconduction velocity of the conduction wave,

identify an origin of an arrhythmia at a location on the surface suchthat a first difference in distances over the surface from the locationto the first pair of the positions is equal to the first LAT-deriveddistance, and a second difference in distances over the surface from thelocation to the second pair of the positions is equal to the secondLAT-derived distance, and

mark the identified origin on the representation of the surface on thedisplay.

In a disclosed embodiment the first pair of the positions and the secondpair of the positions consist of one common position.

In a further disclosed embodiment the processor is configured togenerate a mesh from the location coordinates, and the distances overthe surface include shortest distances measured along edges of the mesh.The mesh may be a triangular mesh.

In a yet further disclosed embodiment the processor may be configured tomark on the representation a first locus of first locations wherein thefirst difference in distances over the surface from the first locationsto the first pair of the positions is equal to the first LAT-deriveddistance, and a second locus of second locations wherein the seconddifference in distances over the surface from the second locations tothe second pair of the positions is equal to the second LAT-deriveddistance. The identified origin is typically at an intersection of thefirst locus and the second locus.

In an alternative embodiment the arrythmia includes a single sourcearrythmia.

In a further alternative embodiment the processor is configured tocalculate the conduction velocity in response to a selected pair ofpoints on the surface, the points being connected by a line segmentparallel to a wavefront vector, and having respective LATs andpositions.

In a yet further alternative embodiment the first difference indistances is equal to the first LAT-derived distance within a presetmargin of error, and the second difference in distances is equal to thesecond LAT-derived distance within the preset margin of error. In oneembodiment the preset margin of error is 1 mm.

An operator of the apparatus may select the first pair of positions andthe second pair of positions.

There is further provided, according to an embodiment of the presentinvention, a method, including:

inserting a probe into a chamber of a heart of a human subject;

generating location signals indicative of location coordinates of adistal end of the probe;

acquiring from positions on a surface of the chamber respectiveelectrical signals in response to a conduction wave traversing thesurface;

processing the location signals to generate and present on a display athree-dimensional (3D) representation of the surface,

deriving respective local activation times (LATs) from the respectiveelectrical signals at the positions on the surface;

calculating a first time difference between the respective LATs at afirst pair of the positions on the surface and a second time differencebetween the respective LATs at a second pair of the positions on thesurface,

calculating first and second LAT-derived distances as respectiveproducts of the first time difference and the second time differencewith a conduction velocity of the conduction wave;

identifying an origin of an arrhythmia at a location on the surface suchthat a first difference in distances over the surface from the locationto the first pair of the positions is equal to the first LAT-deriveddistance, and a second difference in distances over the surface from thelocation to the second pair of the positions is equal to the secondLAT-derived distance; and

marking the identified origin on the representation of the surface onthe display.

The present disclosure will be more fully understood from the followingdetailed description of the embodiments thereof, taken together with thedrawings, in which:

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic illustration of an arrythmia origin locatingsystem, according to an embodiment of the present invention;

FIG. 2 is a schematic diagram illustrating results produced by anelectromagnetic tracking system, according to an embodiment of thepresent invention;

FIG. 3 is a schematic diagram of results generated by anelectrocardiogram module, according to an embodiment of the presentinvention;

FIG. 4 illustrates how a difference in path lengths given by an equationis used for pairs of points selected by a processor, according to anembodiment of the present invention;

FIG. 5 is a flowchart of steps performed by a processor and an operatorin determining a location of an origin of a single-source arrythmiausing the system of FIG. 1, according to an embodiment of the presentinvention;

FIG. 6 illustrates a method for measuring distances to pairs oflocations, according to an embodiment of the present invention;

FIG. 7 is a schematic diagram illustrating, from a conceptual point ofview, how an origin of an arrythmia may be determined, according to anembodiment of the present invention;

FIG. 8 is a schematic diagram illustrating a result obtained by theinventors, according to an embodiment of the present invention; and

FIG. 9 is a diagram of results obtained by the inventors, according toan embodiment of the present invention.

DETAILED DESCRIPTION OF EMBODIMENTS

Overview

Embodiments of the present invention initially generate a map of a heartchamber where a single-source arrythmia is present, and in additionmeasure local activation times (LATs) over the surface of the chamber.The arrythmia is assumed to radiate a conduction wave from an origin, sothat at a given point on the surface of the chamber the LAT for thatpoint is indicative of the time at which the wave from the origin passesthe point.

If a pair of points on the surface is selected, the points willtypically have different LATs, and the difference in the LATs gives ameasure of the difference in distance from the two points to thearrythmia origin. The actual difference in distance is the product ofthe conduction wave velocity and the LAT difference, and this is termedherein an LAT-derived distance. (It will be understood that if the twopoints have the same LATs, they are equidistant from the arrythmiaorigin.)

From the selected pair of points, a locus of possible positions for thearrythmia origin can be found, all the points on the locus having theactual difference in distance to the selected pair that is describedabove.

If another pair of points on the surface is selected the same procedure,to find a second locus of positions for the arrythmia origin, can beapplied to the second pair of points.

The intersection of the loci corresponds to the position of the originof the arrythmia, and this may be displayed on the map. The locithemselves may also be displayed on the map.

It will be understood that the origin location is such that a differencein distances over the surface from the origin location to a first pairof the points is equal to the LAT-derived distance for the first pair,and is also such that a difference in distances over the surface fromthe origin location to a second pair of the points is equal to thesecond LAT-derived distance for the second pair.

Typically, the two pairs of points comprise four physically separateddistinct points (in two pairs). However, in some embodiments one of thepoints is common to both pairs, so that in these embodiments threephysically separated distinct points comprise the two pairs.

System Description

In the following description, like elements in the drawings areidentified by like numerals, and like elements are differentiated asnecessary by appending a letter to the identifying numeral.

Reference is now made to FIG. 1, which is a schematic illustration of anarrythmia origin locating system 20, according to an embodiment of thepresent invention. For simplicity and clarity, the followingdescription, except where otherwise stated, assumes a medical procedureis performed by an operator 22 of system 20, herein assumed to be amedical practitioner, wherein the operator inserts a catheter 24 into aleft or right femoral vein of a patient 28. The procedure is assumed tocomprise investigation of a chamber of a heart 34 of the patient, and inthe procedure, the catheter is initially inserted into the patient untila distal end 32 of the catheter, also herein termed probe 32, reachesthe heart chamber.

System 20 may be controlled by a system processor 40, comprising aprocessing unit (PU) 42 communicating with an electromagnetic trackingmodule 36 and/or a current tracking module 37. PU 42 also communicateswith an ablation module 39 and an ECG (electrocardiograph) module 43.The functions of the modules are described in more detail below. PU 42also communicates with a memory 44. Processor 40 is typically mounted ina console 46, which comprises operating controls 38, typically includinga pointing device such as a mouse or trackball, that operator 22 uses tointeract with the processor. The processor uses software stored inmemory 44 to operate system 20. Results of the operations performed byprocessor 40 are presented to the operator on a display 48, whichtypically presents a map of heart 34. The software may be downloaded toprocessor 40 in electronic form, over a network, for example, or it may,alternatively or additionally, be provided and/or stored onnon-transitory tangible media, such as magnetic, optical, or electronicmemory.

For tracking the path of probe 32 in a mapping region containing heart34, embodiments of the present invention use at least one of a currentbased tracking system 21 and an electromagnetic based tracking system23. Both systems are described below.

Tracking system 21 comprises a current measuring tracking system,similar to that described in U.S. Pat. No. 8,456,182 to Bar-Tal et al.,whose disclosure is incorporated herein by reference. The Carto™ systemproduced by Biosense-Webster of 33 Technology Drive, Irvine, Calif.92618 USA, also uses a current measuring tracking system. The currentmeasuring tracking system is under control of current tracking module37. Probe 32 has one or more probe electrodes 50, and in tracking system21 module 37 injects currents to the one or more electrodes 50 beingtracked. The currents are received, by a plurality of generally similarpatch electrodes 77, also herein termed patches, which are positioned onthe skin of patient 28, and transferred back to the module.

While conductive cabling to patch electrodes 77 and for other skinelectrodes described herein is present for each of the electrodes, forclarity cabling is only shown in the figure for some of the electrodes.The currents between a given probe electrode 50 and skin patches 77 varyaccording to the location of the electrode, because, inter alia, of thedifferent distances of the electrode from the patches, which causedifferent impedances between the given probe electrode and the differentpatches. Module 37 measures the different currents received by thedifferent patches 77 on respective channels connected to the patches,and may be configured to generate an indication of the location of thegiven probe electrode from the different currents.

Electromagnetic tracking system 23 is similar to that described in U.S.Pat. No. 6,690,963 to Ben-Haim et al., whose disclosure is incorporatedherein by reference, and to that used in the Carto™ system produced byBiosense-Webster. The electromagnetic tracking system is under controlof electromagnetic tracking module 36. The electromagnetic trackingsystem comprises a plurality of magnetic field generators, hereinassumed to comprise three sets of generators 66, each set comprisingthree orthogonal coils, so that the plurality of generators comprises atotal of nine coils. Generators 66 are placed in known locations beneathpatient 28, the known locations defining a frame of reference of thegenerators. Module 36 controls, inter alia, the amplitude and frequencyof the alternating magnetic fields produced by the generators.

The alternating magnetic fields interact with a coil located in probe32, so as to generate alternating electropotentials in the coil, and theelectropotentials are received as a signal by tracking module 36. Themodule, together with processing unit 42, analyzes the received signal,and from the analysis is able to determine a position, i.e., a locationand an orientation, of the probe coil in the defined frame of reference.

Typically the tracking by either or both of the systems may be presentedvisually on display 48, for example by incorporating an iconrepresenting the probe into an image of heart 34, as well as a pathtaken by the icon. For clarity, in the following description, onlyelectromagnetic tracking system 23 is assumed to be use, but thedescription may be adapted, mutatis mutandis, for cases where bothsystem 23 and system 21 are used, or if only system 21 is used.

Ablation module 39 comprises a radiofrequency (RF) generator whichdelivers RF power to a region of heart 34 that is selected by operator22, so as to ablate the region. Operator 22 selects the region bypositioning an ablation probe, with an ablation electrode, at theregion. In some embodiments probe 32 and one of electrodes 50 may beused as an ablation probe and an ablation electrode. Alternatively aseparate ablation probe and ablation electrode may be used for theablation provided by module 39.

ECG module 43 receives ECG signals from electrodes 50, and together withPU 42 analyzes the signals to find, inter alia, local activation times(LATs) of the signals. The module typically measures the LAT valuesrelative to a reference ECG signal, such as may be provided by anelectrode positioned in the coronary sinus of heart 34.

FIG. 2 is a schematic diagram illustrating results produced byelectromagnetic tracking system 23, according to an embodiment of thepresent invention. During the procedure referred to above, probe 32 ismoved within heart 34, herein assumed to be within a chamber of theheart, and as it is moved tracking module 36 acquires positional signalsfrom the probe, and uses the signals to find three-dimensional (3D)positions of the probe. The multiple positions found comprise a pointcloud of locations on the surface of the chamber, as well as locationswithin the chamber.

From the point cloud processor 40 generates a triangular mesh of a 3Denclosing surface, i.e., a surface enclosing all the acquired points inthe point cloud, corresponding to the surface of the heart chamber. Theprocessor uses any method known in the art to produce the mesh.Typically processor 40 “covers” the triangular mesh to form a smoothcontinuous 3D surface, and the processor may display a graphicrepresentation 49 of the smooth 3D surface on display 48. In addition,the processor typically covers triangles of the mesh with equally spacedsample points, and these points provide processor 40 with a method toperform discrete calculations on the continuous 3D surface. An exampleof the use of the sample points described herein is provided below, withreference to FIG. 6.

FIG. 2 schematically shows a set of points 80, comprising some of thepoints in the point cloud referred to above, each of the pointscorresponding to respective points on the surface of the heart chamber.The figure also illustrates edges 84 connecting the points, the edgescorresponding to line segments joining the points when processor 40generates the triangular mesh. Points 80 and edges 84 are vertices andsides of triangles, and are herein also termed vertices 80 and sides 84.

FIG. 3 is a schematic diagram of results generated by ECG module 43,according to an embodiment of the present invention. The results aregenerated from ECG signals acquired by probe 32 as the probe contactsthe surface of the heart chamber wherein the probe is moved. The ECGsignals may be acquired concurrently with the positional signalsreferred to above, or alternatively at a different time, and possiblywith a different probe.

The figure illustrates four voltage V vs. time t graphs 90, 92, 94, 96,respectively corresponding to ECG signals acquired at four points 80A,80B, 80C, 80D (FIG. 2) on the surface of the heart chamber. For each setof graphs the ECG module calculates the position (in time) of the localactivation time (LAT), and the LAT value is shown schematically on eachof the graphs as circles 90L, 92L, 94L, 96L. Each of the four graphs hasbeen drawn with the V axis corresponding to a reference time of zeroderived from the reference ECG signal referred to above.

In embodiments of the present invention processor 40 selects pairs ofpoints 80 and calculates the difference in LAT values of the ECG signalsof the points. Thus, in FIG. 3 processor 40 may select points 80A and80B, in which case it finds the difference in LAT values as Δt(AB).Similarly the processor may select points 80C and 80D, in which case itfinds the difference in LAT values as Δt(CD).

The following description assumes that for a general pair of points 80selected by processor 40 the difference in LAT values of the ECG signalsis Δt. Embodiments of the present invention assume that the ECG signalsare generated from a single-source arrythmia, and that the ECG signalstraverse the surface of the heart chamber with a conduction velocity v.In this case, for the general pair of points assumed herein there is adifference in path length ΔP, from the single source to the two pointsof the general pair, given by equation (1):

ΔP=v·Δt  (1)

The difference in path length ΔP is a distance, and is also hereintermed the LAT-derived distance.

FIG. 4 illustrates how the difference in path lengths given by equation(1) is used for specific pairs of points 80 selected by processor 40,according to an embodiment of the present invention. The pointsillustrated in FIG. 4 correspond to points 80A, 80B, 80C, and 80D, (FIG.2) and are herein respectively termed A, B, C, D.

For points A, B, a point Q can be the origin of the single sourcearrythmia if equation (2) is valid.

BQ−AQ=v·Δt(AB)  (2)

where BQ is the distance on the surface of the heart chamber betweenpoints B and Q,

AQ is the distance on the surface of the heart chamber between points Aand Q, and

Δt(AB) is the difference in LAT values between the ECG signals frompoints B and A.

The LAT-derived distance for points A, B is the product vΔt(AB).

It will be understood that to satisfy equation (2) the point Q may be ina plurality of locations i.e., Q may be on a locus, or line, 100, whereany point on the locus obeys equation (2). Line 100 is a curved linethat predicts where an origin of the single source arrythmia may be, andlines such as line 100 are also referred to herein as prediction lines.

For points C, D, a point R can be the origin of the single sourcearrythmia if equation (3) is valid.

DR−CR=v·Δt(CD)  (3)

where DR is the distance on the surface of the heart chamber betweenpoints D and R,

CR is the distance on the surface of the heart chamber between points Cand R, and

Δt(CD) is the difference in LAT values between the ECG signals betweenpoints D and C.

The LAT-derived distance for points C, D is the product vΔt(CD).

As for equation (2), equation (3) generates a locus or line 102, andpoint R may be any point on the locus. Line 102 is also a predictionline for the position of the origin of the single-source arrythmia.

Loci 100 and 102 intersect at a region 104, and since region 104 is onboth loci, region 104 corresponds to a predicted origin of thesingle-source arrythmia generating the ECG signals acquired at points80A, 80B, 80C, and 80D.

FIG. 5 is a flowchart of steps performed by processor 40 and operator 22in determining the location of an origin of a single-source arrythmiausing system 20, according to an embodiment of the present invention.

In an initial step 150 operator 22 inserts probe 32 into a chamber ofheart 34 of the patient. Probe 32 is typically a multi-electrode probehaving electrodes 50, such as the Pentaray probe produced byBiosense-Webster. The operator may also set values of parameters, suchas an assumed conduction velocity in heart 34 and a margin of error tobe used in evaluating results, to be used by the processor in performingits calculations for the origin location.

In one embodiment the conduction velocity v in the heart chamber is setto be 1.0 mm/ms, and an acceptable error E in determining the originlocation is set to be 1 mm. It will be understood that both thesefigures are by way of example, and operator 22 may use other values forthe conduction velocity and for the acceptable error.

Alternatively the conduction velocity may be calculated after steps 154and 156 below have been implemented. In this case a wavefront, of aconduction wave originating from the arrythmia, is mapped from locationsand LAT values of points found in step 156. The conduction velocity maybe calculated from pairs of points that are selected to have respectiveline segments, joining the points, that are parallel to an assumedwavefront vector of the conduction wave.

In a data acquisition step 154, ECG signal data and positional signalsare acquired from each of electrodes 50 of the probe.

In an analysis step 156, processor 40 uses the positional signals tofirst identify locations of points 80, and then construct a 3Dtriangular mesh of the surface of the heart chamber. The mesh isconstructed by joining the points with edges, as described above withreference to FIG. 2, so forming a mesh with triangle vertices 80 andtriangle sides 84. The mesh and/or the surface generated by the mesh maybe presented to operator 22 on display 48.

The processor also analyzes the ECG signals to find the LAT values foreach point 80 on the heart chamber surface contacted by electrodes 50,as described above with reference to FIG. 3.

In a pair selection step 160, a pair of locations on the surface of theheart chamber, i.e., two locations that the processor identifies in step156, are selected. The locations are herein termed X₁, X₂. In thefollowing description the selection is assumed to be performed byoperator 22, typically using the map presented to the operator in step156.

Once the pair has been selected, the processor determines the LAT valuesfor each of the ECG signals of the pair, and then finds the differencein LAT values Δt.

The processor then uses equation (1), with the conduction velocity vvalue set in step 150, to determine a path length difference, theLAT-derived distance, ΔP, from the single source to the selected pair oflocations.

In a plurality of locations generation step 164, the processor findspossible locations for the origin of the single-source arrythmia, hereinassigned the label S, by finding locations of S that satisfy equation(4):

SX ₁ −SX ₂ =v·Δt  (4)

where SX₂ is the distance on the surface of the heart chamber betweenpoints S and X₂, and

SX₁ is the distance on the surface of the heart chamber between points Sand X₁.

The LAT-derived distance for points X₁, X₂ is the product vΔt.

(Equation (4) has the same form as equations (2) and (3); only the pointidentifiers have been changed.)

In order to check if equation (4) is satisfied, processor 40 calculatesvalues of SX₁ and SX₂ separately, as follows:

For point X₁ the processor finds the shortest distances, along sides 84,to each of the vertices 80 acquired in step 156. The processor alsofinds the shortest distances, along sides 84, from X₂ to each of thesevertices. In one embodiment the processor uses Dijkstra's algorithm tofind the shortest distances.

For each of the vertices 80, the processor finds the difference Δ in theshortest distances.

In a comparison step 168, for each of the vertices 80, the processorchecks if the difference Δ is close to the path length difference valueΔP found in step 160. I.e., the processor checks if the expression (5)is correct.

|Δ−ΔP|≤E  (5)

where E is the margin of error set in initial step 156.

If comparison 168 returns negative, the vertex 80 being checked is notconsidered to be a possible origin of the arrythmia. In this casecontrol proceeds to a comparison step 172 where the processor checks ifall the vertices have been checked for the locations selected in step160 d

If comparison step 168 returns positive, the vertex 80 being checked isa possible origin of the arrythmia. In this case processor 40 may markthe vertex on the map presented to the operator in a mark vertex step176, and control continues to comparison step 172.

As stated above, in comparison step 172 the processor checks if allvertices have been checked for the locations selected in step 160. Ifthe comparison returns negative, i.e., all vertices have not beenchecked, control returns to step 164.

If comparison step 172 returns positive, the processor continues to afurther comparison step 180, wherein the processor checks if more thanone pair of locations, chosen in step 160, has been selected andanalyzed in the iteration of steps 164, 168, 176, and 172.

If multiple pairs of locations have not been selected (comparison 180returning negative), i.e., only one pair of locations has been selectedin step 160, then the processor selects another pair of locations, fromthose found in step 156, in a select pair location step 184. Controlthen returns to step 160, where the new location pair is analyzed.

Typically, locations of each pair are selected to be distinct, so thatfor two pairs there are four distinct locations. However, in someembodiments, any two pairs may have one common location, so that inthese cases there are three distinct locations.

If comparison 180 returns positive, then two or more pairs of locationshave been selected and analyzed. For each pair vertices have beenmarked, in step 176, as a locus of possible points for the arrythmiaorigin, and so there are two or more loci marked on the map. Asdescribed above, the intersection of the multiple loci corresponds tothe predicted origin of the arrythmia, so that when comparison 180returns positive, in a final step 188 of the flowchart, processor 40 maymark the loci intersection as the predicted arrythmia origin on the map.

In comparison step 180, the processor may stop the return to step 160once a predetermined number of pairs of locations has been has beenchecked, and the number may be set by operator 22. In one embodiment thepredetermined number of pairs is 10, from 20 locations. Alternatively,rather than the processor checking if multiple pairs have been checked,the operator may stop the return to step 160 once two or more locationpairs have been checked, and invoke final step 188 so that the processormarks the intersection.

The description above has assumed that the processor measures distances,from each location selected in step 160, to triangle vertices alongsides 184. The distances are measured so that the processor can find theshortest distance to a triangle vertex.

FIG. 6 illustrates an alternative method for measuring distances topairs of locations, according to an embodiment of the present invention.Rather than measuring to triangle vertices, the processor may measuredistances from the selected locations, along edges 84, to a sample pointP in a given triangle of the mesh generated in step 156. As describedabove with reference to FIG. 2, processor 40 typically covers trianglesof the triangular mesh with equally spaced sample points. FIG. 6illustrates a triangle 200 of the triangular mesh, and sample points 204within the triangle. An example of the sample points has been assigned alabel P. Thus, in FIG. 6, if point A is one of the selected pair oflocations in step 160, the processor first calculates distances from Ato vertices V1, V2, V3 of triangle 200, along edges 84 of triangles (notshown in the figure) connecting to triangle 200. The processor then sumsthe lengths V1P, V2P, and V3P to find possible distances from A to pointP. The processor performs a similar calculation for point B as the otherlocation of the selected pair.

The description of the flowchart of FIG. 5 may be adapted, mutatismutandis, to use the alternative method for measuring distances to pairsof locations described with reference to FIG. 6, and thus find one ormore sample points corresponding to a predicted origin of the arrythmia.

The inventors have tested an embodiment of the present invention usingdata from patients who have undergone successful ablation. The followingsummarizes the results of the test.

FIG. 7 is a schematic diagram illustrating, from a conceptual point ofview, how an origin of an arrythmia may be determined, according to anembodiment of the present invention. The figure assumes that a selectedpair of locations, such as locations A, B of FIG. 4, have been drawn ona plane surface, herein assumed to be an xy plane. The figure furtherassumes that points A, B are separated by a distance z, and that a pointC in the plane is a possible origin of an arrythmia.

As for the system described above with reference to FIG. 4, there is alocal activation time difference between locations A, B, in the figureassumed to be t. There is thus, again as for the system of FIG. 4, apath difference between CA and CB of CV·t, where CV is the conductionvelocity of the wave from the arrythmia, giving the equation:

CV·t=CA−CB  (6)

It will be appreciated that equation (6) corresponds to equations (2),(3), and (4) above.

As shown in the figure, if C is a vertical distance y above AB, and ahorizontal distance x from B, then both CA and CB can be expressed interms of x, y, and z. The expressions are presented in the figure, andsubstituting them into equation (6) gives

CV·t=√{square root over ((z−x)² +y ²)}−√{square root over (x ² +y²)}  (7)

Assuming CV, t and z are known, equation (7) can be solved for possiblevalues of (x,y) for C, the values (x,y) generating a prediction curve ofpossible positions in the plane of the origin of the arrythmia.

If another pair of locations in the plane is selected, then the otherpair generates a second prediction curve of possible positions of theorigin of the arrythmia in the plane, and the intersection of the twolines corresponds to a predicted position for the origin of thearrythmia in the plane.

FIG. 8 is a schematic diagram illustrating a result obtained by theinventors, according to an embodiment of the present invention.Prediction curves for two pairs of locations were plotted on a 3Delectroanatomic map where premature ventricular contraction (PVC) wasoccurring. One prediction curve comprises sets of dark gray points; asecond prediction curve comprises sets of light gray points. (The actualmap was in color, and the curves were in different colors.)

The intersection of the two curves is marked in the figure by a whiteellipse, corresponding to the predicted origin of the arrythmia. Thetrue origin, corresponding to a region where ablation was performedsuccessfully, is shown in the figure, and is very close to the predictedorigin.

FIG. 9 is a diagram of results obtained by the inventors, according toan embodiment of the present invention. Patients with successfulablation of a focal wavefront, as confirmed by traditional mapping andsuccessful ablation with a focal ablation lesion, were retrospectivelyenrolled. Two or more pairs of prediction curves were generated in eachpatient. For each prediction curve conduction velocity was assessed ineach patient using point pairs parallel to a mapped wavefront vector.

The main outcome was the distance between the predicted and the trueorigins for each prediction curve pair.

The inventors produced prediction curves for 28 cases. As shown in thefigure, the overall results for the 28 intersections gave the distancebetween the predicted and true origins of the wavefront generated by thearrythmia as 6.4±7.8 mm. The figure also gives a breakdown of distancesbetween predicted and true origins for different types of arrythmia.

In the figure “ORT” is orthodromic reciprocating tachycardia, “PVI” ispulmonary vein isolation, “AT” is atrial tachycardia, and “PVC/VT” ispremature ventricular contraction/ventricular tachycardia.

Using univariate analysis, accuracy was found to be related to chamberof origin, conduction velocity, the standard deviation of conductionvelocity measurements, the distance between the point pairs, and thecycle length of the rhythm, but not to the average distance of the pointpairs to the wavefront origin or to the activation timing between thepoint pairs. Using multivariate analysis only chamber of origin wassignificant.

It will be appreciated that the embodiments described above are cited byway of example, and that the present invention is not limited to whathas been particularly shown and described hereinabove. Rather, the scopeof the present invention includes both combinations and subcombinationsof the various features described hereinabove, as well as variations andmodifications thereof which would occur to persons skilled in the artupon reading the foregoing description and which are not disclosed inthe prior art.

We claim:
 1. Medical apparatus, comprising: a probe, configured to beinserted into a chamber of a heart of a human subject, and to generatelocation signals indicative of location coordinates of a distal end ofthe probe, and comprising an electrode at the distal end, which isconfigured to acquire from positions on a surface of the chamberrespective electrical signals in response to a conduction wavetraversing the surface; a display; and a processor, configured to:process the location signals to generate and present on the display athree-dimensional (3D) representation of the surface, derive respectivelocal activation times (LATs) from the respective electrical signals atthe positions on the surface, calculate a first time difference betweenthe respective LATs at a first pair of the positions on the surface anda second time difference between the respective LATs at a second pair ofthe positions on the surface, calculate first and second LAT-deriveddistances as respective products of the first time difference and thesecond time difference with a conduction velocity of the conductionwave, identify an origin of an arrhythmia at a location on the surfacesuch that a first difference in distances over the surface from thelocation to the first pair of the positions is equal to the firstLAT-derived distance, and a second difference in distances over thesurface from the location to the second pair of the positions is equalto the second LAT-derived distance, and mark the identified origin onthe representation of the surface on the display.
 2. The apparatusaccording to claim 1, wherein the first pair of the positions and thesecond pair of the positions comprise one common position.
 3. Theapparatus according to claim 1, wherein the processor is configured togenerate a mesh from the location coordinates, and wherein the distancesover the surface comprise shortest distances measured along edges of themesh.
 4. The apparatus according to claim 3, wherein the mesh comprisesa triangular mesh.
 5. The apparatus according to claim 1, and comprisingthe processor being configured to mark on the representation a firstlocus of first locations wherein the first difference in distances overthe surface from the first locations to the first pair of the positionsis equal to the first LAT-derived distance, and a second locus of secondlocations wherein the second difference in distances over the surfacefrom the second locations to the second pair of the positions is equalto the second LAT-derived distance.
 6. The apparatus according to claim5, wherein the identified origin is at an intersection of the firstlocus and the second locus.
 7. The apparatus according to claim 1,wherein the arrythmia comprises a single source arrythmia.
 8. Theapparatus according to claim 1, and comprising the processor beingconfigured to calculate the conduction velocity in response to aselected pair of points on the surface, the points being connected by aline segment parallel to a wavefront vector, and having respective LATsand positions.
 9. The apparatus according to claim 1, wherein the firstdifference in distances is equal to the first LAT-derived distancewithin a preset margin of error, and wherein the second difference indistances is equal to the second LAT-derived distance within the presetmargin of error.
 10. The apparatus according to claim 9, wherein thepreset margin of error is 1 mm.
 11. The apparatus according to claim 1,wherein an operator of the apparatus selects the first pair of positionsand the second pair of positions.
 12. A method, comprising: inserting aprobe into a chamber of a heart of a human subject; generating locationsignals indicative of location coordinates of a distal end of the probe;acquiring from positions on a surface of the chamber respectiveelectrical signals in response to a conduction wave traversing thesurface; processing the location signals to generate and present on adisplay a three-dimensional (3D) representation of the surface, derivingrespective local activation times (LATs) from the respective electricalsignals at the positions on the surface; calculating a first timedifference between the respective LATs at a first pair of the positionson the surface and a second time difference between the respective LATsat a second pair of the positions on the surface, calculating first andsecond LAT-derived distances as respective products of the first timedifference and the second time difference with a conduction velocity ofthe conduction wave; identifying an origin of an arrhythmia at alocation on the surface such that a first difference in distances overthe surface from the location to the first pair of the positions isequal to the first LAT-derived distance, and a second difference indistances over the surface from the location to the second pair of thepositions is equal to the second LAT-derived distance; and marking theidentified origin on the representation of the surface on the display.13. The method according to claim 12, wherein the first pair of thepositions and the second pair of the positions comprise one commonposition.
 14. The method according to claim 12, and comprisinggenerating a mesh from the location coordinates, and wherein thedistances over the surface comprise shortest distances measured alongedges of the mesh.
 15. The method according to claim 14, wherein themesh comprises a triangular mesh.
 16. The method according to claim 12,and comprising marking on the representation a first locus of firstlocations wherein the first difference in distances over the surfacefrom the first locations to the first pair of the positions is equal tothe first LAT-derived distance, and a second locus of second locationswherein the second difference in distances over the surface from thesecond locations to the second pair of the positions is equal to thesecond LAT-derived distance.
 17. The method according to claim 16,wherein the identified origin is at an intersection of the first locusand the second locus.
 18. The method according to claim 12, wherein thearrythmia comprises a single source arrythmia.
 19. The method accordingto claim 12, and comprising calculating the conduction velocity inresponse to a selected pair of points on the surface, the points beingconnected by a line segment parallel to a wavefront vector, and havingrespective LATs and positions.
 20. The method according to claim 12,wherein the first difference in distances is equal to the firstLAT-derived distance within a preset margin of error, and wherein thesecond difference in distances is equal to the second LAT-deriveddistance within the preset margin of error.
 21. The method according toclaim 20, wherein the preset margin of error is 1 mm.